During the last ten years, primary and secondary (rechargeable) lithium batteries have been the object of considerable research and development. The aim is to develop a low cost battery, with a large energy content and good electrical performance. With this in mind, a large number of battery designs have been developed to comply with various applications such as portable products, un-interruptible power supplies (UPS), batteries for zero-emission and hybrid electric vehicles, and automotive start-light-ignition (SLI).
While the focus to date has been on Li-ion batteries that use liquid electrolytes, this technology's basic design creates problems in terms of packaging, format, size, cost, and safety [1]. Ionically conducting solid materials display many advantages over liquids as electrolytes. Polymers offer some advantages in terms of safety and mechanical characteristics over liquid electrolyte systems, and can also be used with lithium metal anodes [2]. Lithium metal anodes provide the highest theoretical capacity density. The mechanical properties of polymer electrolytes decrease problems that might arise from the formation of dendrites that can occur when using lithium metal as the anode. The problem for polymer electrolytes is their low conductivity at room temperature. To overcome this limitation, many approaches have been proposed such as polymer gel electrolytes formed by the introduction of plasticizers or the addition of small molecule additives into the polymer. More recently, plastic crystal electrolytes have been proposed [3, 4, 5, 6]. With conductivities as high as 10−3 S·cm−1 at room temperature and good mechanical properties, plastic crystal electrolytes are one of the most promising alternatives to liquid or gelled electrolytes. Furthermore, in comparison to polymer electrolytes, the preparation of a plastic crystal electrolyte is very easy, does not require much addition of a lithium salt, and doesn't need any solvent or radiation cross-linking.
Plastic crystals are mesophases formed mainly by quasi-spherical or disk-like molecules exhibiting rotational and/or orientational disorder while retaining the long-range translational order [7]. A result of this type of “disorder” is the high diffusivity and plasticity that enables plastic crystals to compete with other materials with similar mechanical properties such as polymer electrolytes. The potential of these phases as ion-conducting materials became evident in a publication reporting high ionic conductivities for organic salts based on quaternary ammonium salts [8].
More specifically for lithium battery applications, high ionic conductivities have been reported for plastic crystal phases based on succinonitrile doped with certain lithium salts [5,6]. The plastic crystal properties of neat succinonitrile (abbreviated as SCN) have been characterized in some detail previously [9]. Succinonitrile exhibits plastic crystal formation at temperatures between −40° C. and 58° C. [9]. In the liquid and plastic crystal form, succinonitrile exists in rotational isomers: gauche and trans. However, at temperatures below −44° C. only the gauche form exists [10]. When doped with 5 mol % of lithium bis-trifluoromethanesulphonylimide (Li(CF3SO2)2N), the plastic crystal range is reduced to between −34° C. and 49° C. [5]. While doping with 5 mol % of lithium tetrafluoroborate (LiBF4) shifts the plastic crystal phase to between −36° C. and 44° C. [5]. The conductivities of these succinonitrile-lithium salts phases have already been discussed in prior publications [4,5]. Amongst the lithium salts evaluated, Li(CF3SO2)2N and LiBF4 show the highest conductivities with succinonitrile in the crystal plastic form with conductivities above 10−3 S·cm−1 for Li(CF3SO2)2N and 10−4 S·cm−1 for LiBF4 at room temperature [5]. These conductivities are good enough to use these electrolytes in lithium batteries at room temperature. Li(CF3SO2)2N-succinonitrile electrolytes have already been demonstrated and quite good electrochemical performances have been obtained using Li(CF3SO2)2N-succinonitrile with a Li4Ti5O12 anode and either LiFePO4 or LiCoO2 as the cathode material [6]. However, for theses batteries, the voltage output is only about 2 V, and consequently, they can not deliver high energy densities.
Canadian patent application 2,435,218 [12] discloses the use of lithium titanate anodes in electrochemical cells comprising a succinonitrile (NC—CH2—CH2—CN) plastic crystal electrolyte. However, the electrochemical potential of lithium titanate is weak (−1.5 V vs. standard hydrogen electrode) compared to the electrochemical potential of lithium metal (−3.045 V vs. standard hydrogen electrode), therefore electrochemical cells based on lithium titanate are incapable of delivering high energy density. For electrochemical cells incorporating succinonitrile, it was believed that lithium metal, and therefore materials having an electrochemical potential similar to lithium metal, could not be used as the anode due to the possibility of reactivity between —CN group and lithium metal [5], resulting in polymerization of the succinonitrile.
International patent publication WO 2007-012174 discloses that lithium-based anodes having a potential within about 1.3 V of lithium metal may be used with succinonitrile-based plastic crystal electrolytes. While electrochemical devices based on such systems are capable of delivering high energy densities, it would be desirable to deliver high energy densities over a broader potential window, thereby allowing the use of a greater variety of cathodes. International patent publication WO 2007-012174 discloses the use of LiBF4 and (Li(CF3SO2)2N in succinonitrile-based plastic crystal electrolytes for electrochemical devices that are stable up to a potential difference with respect to Li+/Li0 of 3.9 V and 4.5 V, respectively.
There remains a need in the art for improved electrochemical devices that enjoy the benefits of a solid ionic electrolyte while being stable over a broader potential window.